Groundwater contaminants frequently have high volatilities. Examples of contaminants include benzene, toluene, ethylbenzene, xylene (BTEX compounds) and related compounds that result from gasoline spills or leaks as well as trichloroethylene (TCE) and related compounds such as dichloroethylene which are the most frequently encountered groundwater contaminants.
In the evaluation of models and processes for groundwater treatment, TCE can be used as a model compound for the evaluation of the model and has been designated as a priority pollutant by the United States Environmental Protection Agency, TCE and related compounds are only weakly toxic and carcinogenic but their degradation products (especially the vinyl chloride formed under anaerobic conditions) may be serious carcinogens. Unlike BTEX compounds, chlorinated hydrocarbons often cannot serve as the sole source of carbon and energy for most microorganisms but co-metabolism (i.e. co-oxidation) is possible. Co-metabolism results from the expression of nonspecific enzymes that degrade the primary substrate and involves the transformation of a compound that does not supply carbon or energy to the microorganism. Co-metabolism of compounds such as TCE has been found to occur with methane, toluene and NH.sub.4 degrading microorganisms. Best results have been shown using methane. Unfortunately, during co-metabolism by methanotrophs, methane acts as a competitive inhibitor of methane monooxygenase, the enzyme responsible for TCE degradation. Therefore, methane concentrations must be kept relatively low.
Biofilters are beds of peat, compost or other materials that can serve as supports for microorganisms while allowing a gas phase to pass through the bed. The microorganisms may then grade organic or inorganic compounds in the gas phase. In particular, the microorganism are used to treat easily degradable compounds such as BTEX and, recently have been shown to have possible applications in TCE treatment.
Furthermore, past systems used in the degradation of VOC's have been limited as a result of the volatization of the VOC's, leading to unacceptable release of VOC's to the atmosphere. The loss of VOC's or pollutant in the gas exit stream is often a result the aeration required by many reactors. Accordingly, there has been a need for a highly effective process and apparatus for the biological degradation of both non-volatile pollutants such as pentachlorophenol (PCP0 as well as volatile pollutants such as TCE or BTEX compounds in water and, in particular, a bioreactor which minimizes the release of VOC's to the atmosphere.
Still further, there has been a need for a system which enables the treatment of volatile products of anaerobic biodegradation processes and, in particular, the effluent of an anaerobic process. For example, it is known that perchloroethylene cannot be readily degraded aerobically but can be degraded to less chlorinated compounds such as TCE and dichloroethane (DCE) anaerobically but that complete mineralization will not take place anaerobically.